To make a spaced disc coaxial cable suitable for outside use, the compartments within the cable must be watertight. If the outer conductor is punctured at any place, water which enters the punctured compartment must not be able to travel lengthwise along the cable beyond the damaged compartment.
One of the problems in constructing such a coaxial cable is that the mechanical strength and the watertightness is achieved partially at the expense of attenuation. For example, adhesive plastics which produce extremely strong bonds to metal have comparatively high dissipation factor and consequently the dielectric losses are fairly high. These dielectric losses are kept within tolerable limits by having air as the dielectric in between the plastic discs which hold the center conductor coaxial with the tubular outer conductor.
Polyethylene is a plastic which is commonly used for the spacer discs of coaxial cable. "Adhesive polyethylene" is a copolymer of ethylene and monomer containing acrylic acid and is made by the Dow Chemical Company under the trade designations of QX-2375 or SD-449. Another adhesive polyethylene is an ionomer manufactured by DuPont under the trade name of Surlyn. These adhesive materials can form a permanent bond with metals, especially aluminum, when heat and pressure are applied to the interface between the discs and the metallic components of the cable.
Other polyolefins, such as polypropylene can be used, but polyethylene is used in the preferred embodiment. The expression "adhesive polyolefin" as used herein, designates polyolefin which has been treated or combined with other material to give it polar characteristics and much stronger adhesion to metals.
The dissipation factor of adhesive polyolefins is higher than that of ordinary polyolefins which have not been treated to make them more adhesive to metals. Where the term "ordinary polyolefin" is used herein, it designates a polyolefin which has not been treated to increase its adherence to metals; and where the term "polyolefin" is used herein without further designation, the material referred to may be either adhesive or non-adhesive polyolefin.
The advantage of low dissipation factor is especially apparent on larger cables and at high frequencies. This is because in large cables attenuation contribution by conductors is comparatively low and the attenuation caused by dielectric losses is independent of cable size. Also the conductor losses at the higher frequencies increase as the square root of the frequency, while the dielectric losses are directly proportional to the frequency.
This invention obtains greater mechanical strength and greater watertightness between compartments of a coaxial cable without increase in attenuation or with such increase as occurs kept within tolerable limits. Several constructions are illustrated in the drawing and explained in the description of the preferred embodiments of the invention.
By necking down the cross-section of a center conductor at the region of the connection of the spacer discs to the conductor, the impedance at the discs can be made the same as at the air gaps so that the cable can be used to transmit microwave signals as well as TV signals.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.